When someone does a search for an item on the trader, it will give a page of results. For example, do a search for Beguiler Blood and it will show various amounts from different sellers.
If you don't like the competition and want to own the market on Beguiler Blood, break up all your stacks and sell them individually 1 at a time. This way when someone does a search, only yours will show up (assuming you have that many to sell).
People can still use the advance search feature to find other sellers, but you would be surprised how many people are too stupid to do this.
It's also handy if you have an alt with a bank guild just to use as extra storage if you plan to flood the market with something.